Water purification using titanium silicate membranes

ABSTRACT

Aqueous streams containing organic and/or inorganic contaminants are purified by the removal of the contaminant by contacting the aqueous stream with a membrane formed from a porous, crystalline titanium silicate.

FIELD OF THE INVENTION

This invention relates to processes for liquid-liquid separationutilizing membranes formed from crystalline titanium silicate molecularsieves. Removal of contaminant organic and inorganic species fromaqueous streams is advantageously achieved.

BACKGROUND OF THE INVENTION

Since the discovery by Milton and coworkers (U.S. Pat. Nos. 2,882,243and 2,882,244) in the late 1950's that aluminosilicate systems could beinduced to form uniformly porous, internally charged crystals, analogousto molecular sieve zeolites found in nature, the properties of syntheticaluminosilicate zeolite molecular sieves have formed the basis ofnumerous commercially important catalytic, adsorptive and ion-exchangeapplications. This high degree of utility is the result of a uniquecombination of high surface area and uniform porosity dictated by the“framework” structure of the zeolite crystals coupled with theelectrostatically charged sites induced by tetrahedrally coordinatedAl⁺³. Thus, a large number of “active” charged sites are readilyaccessible to molecules of the proper size and geometry for adsorptiveor catalytic interactions. Further, since charge compensating cationsare electrostatically and not covalently bound to the aluminosilicateframework, they are generally base exchangeable for other cations withdifferent inherent properties. This offers wide latitude formodification of active sites whereby specific adsorbents and catalystscan be tailor-made for a given utility.

In the publication “Zeolite Molecular Sieves”, Chapter 2, 1974, D. W.Breck hypothesized that perhaps 1,000 aluminosilicate zeolite frameworkstructures are theoretically possible, but to date only approximately150 have been identified. While compositional nuances have beendescribed in publications such as U.S. Pat. Nos. 4,524,055; 4,603,040;and 4,606,899, totally new aluminosilicate framework structures arebeing discovered at a negligible rate.

With slow progress in the discovery of new aluminosilicate basedmolecular sieves, researchers have taken various approaches to replacealuminum or silicon in zeolite synthesis in the hope of generatingeither new zeolite-like framework structures or inducing the formationof qualitatively different active sites than are available in analogousaluminosilicate based materials.

It has been believed for a generation that phosphorus could beincorporated, to varying degrees, in a zeolite type aluminosilicateframework. In the more recent past (JAC 104, pp. 1146 (1982);proceedings of the 7^(th) International Zeolite Conference, pp. 103-112,1986) E. M. Flanigan and coworkers have demonstrated the preparation ofpure aluminophosphate based molecular sieves of a wide variety ofstructures. However, the site inducing Al⁺³ is essentially neutralizedby the p₊₅, imparting a +1 charge to the framework. Thus, while a newclass of “molecular sieves” was created, they are not zeolites in thefundamental sense since they lack “active” charged sites.

Realizing this inherent utility limiting deficiency, for the past fewyears the research community has emphasized the synthesis of mixedaluminosilicate-metal oxide and mixed aluminophosphate-metal oxideframework systems. While this approach to overcoming the slow progressin aluminosilicate zeolite synthesis has generated approximately 200 newcompositions, all of them suffer either from the site removing effect ofincorporated P⁺⁵ or the site diluting effect of incorporatingeffectively neutral tetrahedral +4 metal into an aluminosilicateframework. As a result, extensive research has failed to demonstratesignificant utility for any of these materials.

A series of zeolite-like “framework” silicates have been synthesized,some of which have larger uniform pores than are observed foraluminosilicate zeolites. (W. M. Meier, Proceedings of the 7^(th)International Zeolite Conference, pp. 13-22 (1986)). While thisparticular synthesis approach produces materials which, by definition,totally lack active, charged sites, back implantation after synthesiswould not appear out of the question although little work appears in theopen literature on this topic.

Another and most straightforward means of potentially generating newstructures or qualitatively different sites than those induced byaluminum would be the direct substitution of some charge inducingspecies for aluminum in a zeolite-like structure. To date the mostnotably successful example of this approach appears to be boron in thecase of ZSM-5 analogs, although iron has also been claimed in similarmaterials. (EPA 68,796 (1983), Taramasso, et. al.; Proceedings of the5^(th) International Zeolite Conference; pp. 40-48 (1980)); J. W. Ball,et. al.; Proceedings of the 7^(th) International Zeolite Conference; pp.137-144 (1986); U.S. Pat. No. 4,280,305 to Kouenhowen, et. al.Unfortunately, the low levels of incorporation of the speciessubstituting for aluminum usually leaves doubt if the species areoccluded or framework incorporated.

In 1967, Young in U.S. Pat. No. 3,329,481 reported that the synthesis ofcharge bearing (exchangeable) titaniumsilicates under conditions similarto aluminosilicate zeolite formation was possible if the titanium waspresent as a “critical reagent” +III peroxo species. While thesematerials were called “titanium zeolites” no evidence was presentedbeyond some questionable X-ray diffraction (XRD) patterns and his claimhas generally been dismissed by the zeolite research community. (D. W.Breck, Zeolite Molecular Sieves, p. 322 (1974); R. M. Barrer,Hydrothermal Chemistry of Zeolites, p. 293 (1982); G. Perego, et. al.,Proceedings of 7^(th) International Zeolite conference, p. 129 (1986)).For all but one end member of this series of materials (denoted TSmaterials), the presented XRD patterns indicate phases too dense to bemolecular sieves. In this case of the one questionable end member(denoted TS-26), the XRD pattern might possibly be interpreted as asmall pored zeolite, although without additional supporting evidence, itappears extremely questionable.

A naturally occurring alkaline titanosilicate identified as “Zorite” wasdiscovered in trace quantities on the Siberian Tundra in 1972 (A. N.Mer'kov, et. al.; Zapiski Vses Mineralog. Obshch., pp. 54-62 (1973)).The published XRD pattern was challenged and a proposed structurereported in a later article entitled “The OD Structure of Zorite”,Sandomirskii, et. al., Sov. Phys. Crystallogr. 24(6), November-December1979, pp. 686-693.

No further reports on “titanium zeolites” appeared in the openliterature until 1983 when trace levels of tetrahedral Ti(IV) werereported in a ZSM-5 analog. (M. Taramasso, et. al.; U.S. Pat. No.4,410,501 (1983); G. Perego, et. al.; Proceedings of the 7^(th)International Zeolite Conference; p. 129 (1986)). A similar claimappeared from researchers in mid-1985 (EPA 132,550 (1985)). The researchcommunity reported mixed aluminosilicate-titanium (IV) (EPA 179,876(1985); EPA 181,884 (1985) structures which, along with TAPO (EPA121,232 (1985) systems, appear to have no possibility of active titaniumsites. As such, their utility has been limited to catalyzing oxidation.

In U.S. Pat. No. 4,938,939, issued Jul. 3, 1990, Kuznicki disclosed anew family of synthetic, stable crystalline titanium silicate molecularsieve zeolites, which have a pore size of approximately 3-4 Angstromunits and a titania/silica mole ratio in the range of from 1.0 to 10.The entire content of U.S. Pat. No. 4,938,939 is herein incorporated byreference. Members of the family of molecular sieve zeolites designatedETS-4 in the rare earth-exchanged form have a high degree of thermalstability of at least 450° C. or higher depending on cationic form, thusrendering them effective for use in high temperature catalyticprocesses. ETS zeolites are highly adsorptive toward molecules up toapproximately 3-5 Angstroms in critical diameter, e.g. water, ammonia,hydrogen sulfide, SO₂, and n-hexane and are essentially non-adsorptivetoward molecules, which are larger than 5 Angstroms in criticaldiameter.

A large pore crystalline titanium silicate molecular sieve compositionhaving a pore size about 8 Angstrom units has also been developed by thepresent assignee and is disclosed in U.S. Pat. No. 4,853,202, whichpatent is herein incorporated by reference. This crystalline titaniumsilicate molecular sieve has been designated ETS-10.

The new family of microporous titanium silicates developed by thepresent assignee, and generically denoted as ETS, are constructed fromfundamentally different building units than classical aluminosilicatezeolites. Instead of interlocked tetrahedral metal oxide units as inclassical zeolites, the ETS materials are composed of interlockedoctahedral chains and classical tetrahedral rings. In general, thechains consist of six oxygen-coordinated titanium octahedra and whereinthe chains are connected three dimensionally via tetrahedral siliconoxide units or bridging titanosilicate units. The inherently differentcrystalline titanium silicate structures of these ETS materials havebeen shown to produce unusual and unexpected results when compared withthe performance of aluminosilicate zeolite molecular sieves. Forexample, the counter-balancing cations of the crystalline titaniumsilicates are associated with the charged titania chains and not theuncharged rings, which form the bulk of the structure. In ETS-10, thisassociation of cations with the charged titania chains is widelyrecognized as resulting in the unusual thermodynamic interactions with awide variety of sorbates, which have been found. This includes relativeweak binding of polar species such as water and carbon dioxide andrelatively stronger binding of larger species, such as propane and otherhydrocarbons. These thermodynamic interactions form the heart of lowtemperature desiccation processes as well as evolving Claus gaspurification schemes. The unusual sorbate interactions are derived fromthe titanosilicate structure, which places the counter-balancing cationsaway from direct contact with the sorbates in the main ETS-10 channels.

In recent years, scores of reports on the structure, adsorption and,more recently, catalytic properties of wide pore, thermally stableETS-10 have been made on a worldwide basis. This worldwide interest hasbeen generated by the fact that ETS-10 represents a large pore thermallystable molecular sieve constructed from what had previously been thoughtto be unusable atomic building blocks.

Although ETS-4 was the first molecular sieve discovered which containedthe octahedrally coordinated framework atoms and as such was consideredan extremely interesting curiosity of science, ETS-4 has been virtuallyignored by the world research community because of its small pores andreported low thermal stability. As synthesized, ETS-4 has anapproximately 4 Å effective pore diameter. Reference to pore size or“effective pore diameter” defines the effective diameter of the largestgas molecules significantly adsorbed by the crystal. This may besignificantly different from, but systematically related to, thecrystallographic framework pore diameter. For ETS-4, the effective poreis defined by eight-membered rings formed from TiO₆ ²− octahedra andSiO₄ tetrahedra. This pore is analogous to the functional pore definedby the eight-membered tetrahedral metal oxide rings in traditionalsmall-pored zeolite molecular sieves.

The pores of ETS-4 formed by the eight-membered polyhedral TiO₆ and SiO₄units are non-faulted in a singular direction, the b-direction, of theETS crystal and, thus, fully penetrate the crystal, rendering the ETS-4useful for molecular separations. Recently, however, researchers of thepresent assignee have discovered a new phenomenon with respect to ETS-4.In appropriate cation forms, the pores of ETS-4 can be made tosystematically shrink from slightly larger than 4 Å to less than 3 Åduring calcinations, while maintaining substantial sample crystallinity.These pores may be “frozen” at any intermediate size by ceasing thermaltreatment at the appropriate point and returning to ambient temperature.These materials having controlled pore sizes are referred to as CTS-1(contracted titanosilicate-1) and are described in commonly assignedU.S. Pat. No. 6,068,682, issued May 30, 2000 herein incorporated byreference in its entirety. Thus, ETS-4 may be systematically contractedunder appropriate conditions to CTS-1 with a highly controllable poresize in the range of 3 -4 Å. With this extreme control, molecules inthis range may be separated by size, even if the sizes of the respectivemolecules are nearly identical. This profound change in adsorptivebehavior is accompanied by systematic structural changes as evidenced byX-ray diffraction patterns and infrared spectroscopy. The systematiccontraction of ETS-4 to CTS-1 to a highly controllable pore size hasbeen named the Molecular Gate™ effect. This effect is leading to thedevelopment of separation of molecules differing in size by as little as0.1 Angstrom, such as N₂/O₂ (3.6 and 3.5 Angstroms, respectively),CH₄/N₂ (3.8 and 3.6 Angstroms), or CO/H₂ (3.6 and 2.9 Angstroms). Highpressure N₂/CH₄ separation systems are now being developed.

Separations of fluid mixtures (gases or liquids) by adsorption utilizingthe ETS-type molecular sieves have been proposed in which the molecularsieve is utilized in the form of a bed, typically fixed, through whichthe mixture to be separated flows. Both pressure swing adsorption (PSA)and thermal swing adsorption (TSA) have been suggested to effectseparation of one or more fluids from mixtures containing same.Presently suggested separations using ETS molecular sieve adsorbentsinclude the use of ETS-10 to adsorb hydrocarbon species from a Clausfeed gas also containing hydrogen sulfide and other polar gases. In suchprocess, the ETS-10 adsorbent is regenerated by a temperature swing(TSA) causing desorption of the hydrocarbons. Also proposed by thepresent assignee is the use of ETS-4 and contracted versions thereof,CTS-1, in a high pressure separation of nitrogen from natural gas. Inthis latter system, pressure swing adsorption (PSA) is utilized toadsorb the nitrogen from the natural gas stream, and desorb the nitrogenfrom the titanium silicate molecular sieve.

The unique property of ETS-10 to only weakly bind polar species so as tocause polar species to pass through the adsorbent at mildly elevatedtemperatures, and the ability to actually control and systematicallyshrink the pore size of ETS-4 to its CTS version have playedsignificantly in allowing high capacity, fixed bed separation systems tobe developed utilizing these titanium silicate molecular sieves. Onedisadvantage, however, of the PSA and TSA systems is that the adsorbentbeds quickly reach the sorbent capacities thereof resulting in a“breakthrough” of the sorbate into the product stream. An additionaldisadvantage of these processes is that at elevated temperatures, theadsorbent bed loses its capacity to hold the sorbate, resulting in thecontamination of the product stream as the non-adsorbed sorbate passesthrough the spaces between the individual particles of the molecularsieve and breaks through into the product. Heating, however, is oftenadvantageous to improve the kinetics of the adsorption process.Accordingly, adsorption undertaken in the presence of a bed of molecularsieve operates under multiple timed cycles of adsorption and desorptionto prevent over-reaching the capacity of the adsorbent and consequentbreakthrough of the sorbate into the product during adsorption, andcontamination of the sorbate by product fronts during desorption. On adaily basis, many of such adsorption/desorption cycles must be run.Typically, multiple beds of molecular sieve are used, operating inparallel, some of which are undergoing adsorption while others undergodesorption or intermediate pressurizations and depressurizations if aPSA system is utilized. The need for multiple cycles and/or multiplebeds obviously requires a high capital investment for production of asignificant volume of product such as on a commercial scale. Moreover,such systems have high maintenance costs.

The use of membranes to provide fluid separation of mixtures is a knownalternative to the use of beds of molecular sieves and use thereof inPSA or TSA processes. The membrane separation process is rather straightforward and does not require the timed cycles of adsorption anddesorption needed with fixed bed molecular sieve technology. In membraneapplications, small molecules (permeate) are not adsorbed but simplypass across the plane of the membrane through distinctly sized pores.The larger sized molecules (retentate) cannot pass through the pores andare retained upstream of the membrane plane. Accordingly, there is noadsorbent over-capacity problem and consequent breakthrough of retentateinto product, and, thus, no need for timed cycles. There are, however,disadvantages to membrane technology. For one, while advances in polymermembranes have been made, these materials are still subject to chemicaldestabilization and are not universally inert to all fluid mixtures.Even water present can degrade many such membranes. Zeolite crystallinealuminosilicates with a narrow distribution of pore sizes on a molecularscale have high thermal, chemical, and mechanical stabilities. Molecularsieves can be, for example, alumina phosphates (ALPO) orsilicoaluminophosphates (SAPO), which are also microporous, crystallinematerials with a narrow distribution of pore sizes and also have highthermal, chemical, and mechanical stabilities. Therefore, zeolites andmolecular sieves can be used in bed form not only for fluid separationsin adsorption/desorption processes, as mentioned above, but also asdiffusion membranes when prepared in thin film form. The size andadsorption properties of the zeolite pores, however, limit what can beseparated with a particular type of zeolite membrane, even if thecrystalline structure is perfect and defect free. Zeolite membranes arefurther problematic with respect to polar species, which are stronglyheld within the charged structure of the zeolite pores. Thus, fluidmixtures containing water, CO₂, etc. can adversely affect membraneproductivity. The simplification and, thus, lower capital andoperational costs of separations utilizing membranes, however, whereinthe permeate and retentate are continuously separated is a large factorin the continued development of membrane separations.

SUMMARY OF THE INVENTION

In accordance with this invention, separation of components from gaseousor liquid mixtures containing same is provided by contacting themixtures with membranes formed from titanium silicate molecular sieves,including the ETS molecular sieves developed by Engelhard Corporation.The ETS sieves are distinguished from other molecular sieves bypossessing octahedrally coordinated titania active sites in thecrystalline structure. These molecular sieves contain electrostaticallycharged units that are radically different from charged units inconventionally tetrahedrally coordinated molecular sieves such as in theclassic aluminosilicate zeolites. Members of the ETS family of sievesinclude, by way of example, ETS-4 (U.S. Pat. No. 4,938,939), ETS-10(U.S. Pat. No. 4,853,202), and ETAS-10 (U.S. Pat. No. 5,244,650), all ofwhich are titanium silicates or titanium aluminum silicates. Thedisclosures of each of the listed patents are incorporated herein byreference.

Membranes formed from ETS-4 molecular sieve are particularly usefulinasmuch as the pores of the ETS-4 membranes can be systematicallycontracted under thermal dehydration to form CTS-1-type materials asdisclosed in U.S. Pat. No. 6,068,682. Under thermal dehydration, thepore size of ETS-4 can be systematically controlled from about 4 Å to2.5 Å and sizes therebetween and frozen at the particular pore size byending the thermal treatment and returning the molecular sieve toambient temperature. The ability to actually control the pore size of aparticular molecular sieve greatly increases the number of separationsachievable by a single molecular sieve unlike previous zeolite membranesin which the adsorption and diffusion properties of the zeolite poreslimit what can be separated with a particular type of zeolite membrane.It has recently been discovered that certain polymorphs of ETS-4 can beprepared which not only contain the open small pores along the b-axis ofthe crystallographic lattice, which characterize ETS-4, but whichfurther contain larger pores which are open and interpenetrate thelattice in the c-direction. Controlled shrinkage of these larger poresfurther increases the number of molecules which an be separated by thispolymorph-enriched ETS-4. This material has been called ETS-6 and is thesubject of co-pending application U.S. Ser. No. 09/640,313.

The titanium silicate membranes of this invention can be prepared bymethods known in the art, such as by processes used for preparingaluminosilicate zeolite membranes. Novel methods of forming themembranes are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of a process using titanium silicate membranes forseparating nitrogen from natural gas.

FIG. 2 is a depiction of a separation process using titanium silicatemembranes for enriching air with oxygen.

FIG. 3 is a depiction of a separation process using titanium silicatemembranes for removing organic compounds from water.

DETAILED DESCRIPTION OF THE INVENTION

It is preferred to use as the titanium silicate material which is formedinto membranes of this invention, ETS-4, and more preferably, contractedversions of ETS-4, known as CTS-1. ETS-4 titanium silicates have adefinite X-ray diffraction pattern unlike other molecular sieve zeolitesand can be identified in terms of mole ratios of oxides as follows:

1.0±0.25M_(2/n)O:TiO₂:ySiO₂:zH₂O

wherein M is at least one cation having a valence of n, y is from 1.0 to10.0, and z is from 0 to 100. In a preferred embodiment, M is a mixtureof alkali metal cations, particularly sodium and potassium, and y is atleast 2.5 and ranges up to about 5. The original cations M can bereplaced at least in part with other cations by well-known exchangetechniques. Preferred replacing cations include hydrogen, ammonium,alkaline earth, rare earth, and mixtures thereof.

Members of the ETS-4 molecular sieve zeolites have an orderedcrystalline structure and an X-ray powder diffraction pattern having thefollowing significant lines:

TABLE 1 XRD PQWDER PATTERN OF ETS-4 (0-40° 2 theta) SIGNIFICANTd-SPACING (ANGS.) I/I₀ 11.65 ± 0.25  S-VS 6.95 ± 0.25 S-VS 5.28 ± 0.15M-S 4.45 ± 0.15 W-M 2.98 ± 0.05 vS In the above table, VS = 50-100 S =30-70 M = 15-50 W = 5-30

The above values were collected using standard techniques on a PhillipsAPD3720 diffractometer equipped with a theta compensator as described byaforementioned U.S. Pat. No. 4,938,939, incorporated by reference.

As disclosed in commonly assigned U.S. Pat. No. 6,086,682, it has beendiscovered that ETS-4 can be transformed into CTS-1 by the heating ofETS-4, preferably in the strontium or calcium form with or without lowlevels of sodium, at temperatures ranging from about 50° C. to 450° C.,or preferably 200° C. to 350° C. for strontium and/or calcium mixed withsodium for 0.5 to 100 or more hours, preferably 24-48 hours, thencooling the thus treated material in order to lock in the desired poresize. The manner of cooling is not critical and it can be accomplishedin air, vacuum or inert gas either slowly or rapidly. The calcinationtemperature used to achieve a desired pore diameter depends on thecations present in the reagent ETS-4. Although multivalent strontium andcalcium are the preferred cations for CTS-1 for natural gas separations,other cations can be used with appropriate changes of temperature andduration of thermal treatment. Various combinations of Sr, Ca, Li, Mg,Na, H, Ba, Y, La, and/or Zn have all demonstrated N₂/CH₄ selectivities.

Although the novel materials of this invention have been prepared fromthe calcium and strontium form of ETS-4, there is no theoretical reasonwhy other cations could not be used with appropriate changes oftemperature and duration of thermal treatment. Additionally, the CTS-1materials can be back-exchanged with metal, ammonium or hydrogen ions ina conventional manner if such is desired.

The crystalline titanium molecular sieves hereafter referred to asCTS-1, have a pore size of about 3-4 Å and have a composition in termsof mole ratio of oxides as follows:

1.0±0.25M_(2/n)O:TiO₂:ySiO₂:zH₂O

where M is at least 1 cation having a valence n, y is 1-10 and z is from0-10 and more preferably 0-5, and characterized by an X-ray powderdiffraction pattern having the lines and relative intensity set forth inTable 2 below.

TABLE 2 XRD POWDER PATTERN OF CTS-1 (0-40° 2 theta) SIGNIFICANTd-SPACING (ANGS.) I/I₀ 11.4 ± 0.25 Very Strong 6.6 ± 0.2 Medium-Strong 4.3 ± 0.15 Medium-Strong  3.3 ± 0.07 Medium-Strong 2.85 ± 0.07Medium-Strong

wherein very strong equals 100, medium-strong equals 15-80.

A particularly useful titanium silicate is ETS-4 which is enriched withone or two polymorphs having small pores interpenetrating thecrystallographic b-axis of ETS-4 and larger pores (6 Å) interpenetratingthe crystallographic c-axis. This material has been given the name ETS-6and has a composition in terms of mole ratios of oxides as follows:

1.0+0.25M_(2/n)O:TiO₂:ySiO₂:zH₂O

wherein M is at least one cation having a valence of n, y is from 1.0 to100, and z is from 0 to 100 and characterized by an X-ray powderdiffraction pattern having the lines and relative intensities set forthin Table 3 below:

TABLE 3 XRD POWDER PATTERN OF ETS-6 (0-40° 2 theta) SIGNIFICANTd-SPACING (ANGS.) I/I₀ 12.50 ± 0.25 W-M 11.65 ± 0.25 S-VS  6.95 ± 0.25S-VS where, VS = 50-100 S = 30-70 M = 15-50 W = 5-30

ETS-6 is disclosed in U.S. Ser. No. 09/640,313, the entire contents ofwhich are herein incorporated by reference. Other titanium silicates canbe formed into membranes and used in fluid (gas and liquid) separationin accordance with this invention, including ETS-10 and ETAS-10.

The titanium silicate molecular sieves can be produced as membranes byany technique known in the art, including methods known in the art forproduction of zeolite membranes. The membranes can be unsupported orsupported on a porous metal or ceramic and the like. For example, thetitanium silicate membranes can be formed from hydrothermal synthesisusing aqueous solutions of the titanium silicate precursors spreadagainst a substrate surface to form the membrane layer. Likewise, gelsof the titanium silicate precursors can be spread across a surface andthe gel precursors again heat treated to form the appropriate titaniumsilicate molecular sieve. Growth from solid precursors, such as shapedTiO₂ can be performed. Other methods include chemical vapor depositionwhich is also known in the art. Reference is made to U.S. Pat. No.6,051,517 which sets forth numerous articles describing the preparationof zeolite membranes as well as U.S. Pat. Nos. 5,110,478; 5,100,596;5,069,794; 5,019,263; 4,578,372; 4,699,892, all of which describezeolite membrane preparation and are incorporated herein by reference.For this invention, the particular membrane-forming method is notbelieved to be critical. Any method can be used so long as the membranesare relatively free of defects so as to prevent passage of retentateacross the membrane.

The present invention is also directed to a novel method of forming thetitanium silicate membranes. Thus, it is preferred to form the titaniumsilicate membranes in-situ directly from TiO₂ or mixtures of TiO₂ withother species such as SiO₂. In this particular process, the solidtitanium dioxide is simply immersed in or otherwise continuouslycontacted with an aqueous solution containing the other synthesiscomponents including sodium silicate, sodium hydroxide and water and themixture reacted at elevated temperatures sufficient for synthesis of thedesired molecular sieve. A portion of the silicate reactant can also beincluded in solid form. It may also be useful to include seeds of thecrystalline titanium silicate molecular sieve, which is to be formedinto a membrane in the solid mix or added as a slurry to the liquidsynthesis solution containing the other components.

This novel membrane-forming method is particularly useful inasmuch asthe solid components such as the titanium dioxide, and any otheroptional solids such as silica or any seed titanium silicate crystals,can be pre-formed into any desired shape such as being pressed underheavy pressure and into consolidated thin films. Once the solid isformed into the desired shape, such as a desired thin film, the shapedtitanium dioxide solid and optional solid components can be immersed inan aqueous solution containing the other components present in thecorrect amounts for the synthesis of the desired molecular sieve.Subsequent heating, typically at an elevated temperature such as between150° C. and 350° C., preferably between 200° and 225° C. for 8 to 24hours results in the in-situ formation of the desired crystallinetitanium silicate molecular sieve in the form of the pressed preformsuch as a thin film membrane. When performing a seeded synthesis inwhich the solid titanium dioxide preform also includes a portion of thecrystalline titanium silicate molecular sieve which is desired to beformed, anywhere from 1 to 40% by weight of the preformed consolidatedsolids can be the crystalline titanium silicate seed. Preferably, fromabout 5 to up to about 25 wt. % of the solid preform is the titaniumsilicate seed crystals. The crystalline titanium silicate molecularsieves, including ETS-4, have a density which is much lower thantitanium dioxide. Accordingly, the titanium silicate crystals growoutward to fill gaps in the growing membrane. Fissures and otherimperfections in the membrane can thereby be greatly minimized. While itis preferred to preform the titanium dioxide into thin films such as foruse in membrane film separation, any size or shape membrane can begenerated by preforming the precursor solid mixture including titaniumdioxide and optional solids such as silica and titanium silicate seedcrystal.

The titanium silicate molecular sieve membranes of the present inventioncan be utilized in any process known to separate one molecule fromanother whether in the liquid or gaseous state. In the preferredembodiments of this invention, ETS-4 and its contracted versions, CTS-1,are used in the form of membranes to separate gaseous molecules whichhave a size difference of 0.1 Å or more. This is possible since the poresize of the CT-1 material can be precisely controlled by shrinking thepores of ETS-4 upon thermal treatment. The pores of the CTS-1 materialcan be frozen in place at a particular size by stopping the thermaltreatment and cooling to ambient temperature.

One particular preferred use of the titanium silicate molecular sievemembranes, and, in particular, the CTS-1 membranes, is the separation ofsmall polar species such as CO₂, H₂O, N₂, H₂S and even SO₂ fromhydrocarbons such as raw natural gas at mildly elevated temperature andfull natural gas pressure. In 1993, the Gas Research Institute (GRI)estimated that 10-15% (about 22 trillion cubic feet) of the natural gasreserves in the U.S. are defined as sub-quality due to the contaminationwith nitrogen, carbon dioxide, and sulfur. Nitrogen and carbon dioxideare inert gases with no BTU value and must be removed to low levels,i.e. less than 4%, before the gas can be sold. The purification ofnatural gas usually takes place in two stages in which the polar gasessuch as CO₂, H₂S and water are removed prior to nitrogen removal.Generally, CO₂, H₂S, and H₂O removal are currently performed using threeseparate systems including acid gas scrubbers for removal of H₂S, SO₂and CO₂, glycol dehydration, and molecular sieve dehydration. Atpresent, nitrogen removal is typically limited to cryogenics. Acryogenic process is expensive to install and operate, limiting itsapplication to a small segment of reserves. For example, a nitrogencontent of higher than 15% is needed to render the process economical.While proposed pressure swing adsorption processes utilizing titaniumsilicate molecular sieves are being developed by the present assignee,such processes up to this time have also relied on removal of polargases prior to contacting the nitrogen-containing natural gas with theadsorbent. Further, as previously described, these PSA systems arecostly to build and to operate.

It is believed possible that with the titanium silicate membranes ofthis invention, in particular, with the use of CTS-1 membranes, naturalgas purification can be substantially accomplished in one step. This onestep process is depicted in FIG. 1. As shown therein, a raw natural gasstream is directed to a CTS-1 membrane which has been formed fromthermally treated ETS-4 or an ETS-4 membrane with appropriate cation soas to have a pore size in the range of from 3.6-3.7 Å. The raw naturalgas stream, methane, contains impurities in the form of polar speciessuch as carbon dioxide, hydrogen sulfide, water, and other impuritiessuch as nitrogen and higher hydrocarbons such as C₂-C₇ alkanes. Methane,which has a size of 3.8 Å and the higher hydrocarbons cannot passthrough the pores of the CTS membrane and form the retentate product.The polar species, smaller than methane, have a size of at most 3.6 Åand are able to pass through the pores and across the plane of themembrane as permeate product. It is preferred to heat the gas attemperatures of about 50-125° C., as it has been found that at thesemildly elevated temperatures, the polar species more readily passthrough the pores of the titanium silicate membrane. As can be seen inFIG. 1, the use of the titanium silicate membrane purifies natural gas(methane) in one step, wherein the polar species and other low heatvalue materials (N₂) can be made to pass through the membrane bycontrolling the pore size thereof and removed as permeate by methodsknown in the art of membrane technology. The product natural gas streamcan be collected as retentate and removed from the upstream side of themembrane, again, by methods known in the membrane technology.

Another important use of the titanium silicate membranes of thisinvention is in the formation of enriched oxygen streams with an oxygenpurity of at least 30%. Such oxygen enriched streams could be used in avariety of applications including, for example, providing economical,transportable medical oxygen for those with respiratory difficulty, aswell as in various combustion applications including diesel engines,enabling a cleaner, more economical combustion process. Enriched oxygencould also lead to more efficient production of pure oxygen and nitrogenby pre-enriching the air streams that enter conventional cryogenicoxygen plants. Dramatic increases in combustion engine performance arepossible given the cost effective approach to oxygen enrichment.Expansion of on-site oxygen applications, such as enhancedengine/furnace combustion, will require a dramatic change in airseparation technology. Thermodynamically driven N₂ selective adsorbentshave not proven fast enough for such applications.

Current technology for providing enriched oxygen streams includescryogenics, PSA, vacuum swing adsorption, VSA and VPSA technology aswell as nitrogen membranes. Cryogenic distillation produces very pure(99.999% pure) oxygen (in either liquid or gaseous form) that is thenback diluted with air. The high capital and operating cost of cryogenicsmakes this applicable only in high purity, high pressure or high volume(greater than 150 tons per day) uses. Current VSA technology, usingcarbon molecular sieves, can produce enriched oxygen at costsapproaching $25/ton but rate of oxygen generation and robustness of theadsorbent limit its applications.

Nitrogen membranes can also be used to produce enriched oxygen streamswith oxygen purities of 40%, however, these units suffer from fluxlimitations as well as contamination issues that restrict their use tolower volume applications. Membranes must also be kept free of moisturewhich requires some pre-drying or heating of the air prior to enteringthe membrane systems. The titanium silicate molecular sieve membranes ofthis invention have great potential value in providing a simplifiedmethod of forming enriched oxygen streams.

Air separation utilizing the CTS titanium silicate membranes of thisinvention is shown in FIG. 2. It is important to note that since polarmolecules such as water bind much more weakly on the titanosilicatemolecular sieves than classical zeolites, these polar species arerelatively mobile at mildly elevated temperatures of 50-125° C.Accordingly, raw, humid air can be treated with the titanium silicatemembranes such as of the CTS type. Thus oxygen and water readilypenetrate a CTS pore of approximately 3.5-3.6 Å as permeate while thepassage of the larger N₂ species is prevented by size exclusion. Themembrane separation process of this invention is a one-step separationof air without pre-drying as is the case with many polymeric membraneswhich can be degraded by moisture. Referring to FIG. 2, it can be seenthat raw air can be divided between its larger nitrogen component havinga size of 3.6 Å as retentate from the smaller oxygen at 3.5 Å and polarwater at 2.7 Å which are separated as permeate product. The CTS membraneis formed from ETS-4 by thermal treatment to provide a pore size of3.5-3.6 Å.

The titanium silicate membranes of this invention can be used in manyother separation processes. The membranes can be used in the removal ofargon from oxygen, sulfur dioxide removal from refinery gas or wet gasstreams including raw stack gas or natural gas stream. Additionally, theprecisely tailorable pore size of the CTS membrane would appear to beuseful for separation of small hydrocarbons. Further separationsinclude, in general, separation of mixtures including one or moremolecular species having a diameter of 3 or more Angstroms from one ormore molecular species having diameters of less than 3 Angstroms.Separation of water from air (O₂ and/or N₂), hydrogen from carbonmonoxide and/or carbon dioxide, ammonia from hydrogen sulfide, etc. area few non-limiting examples.

While the titanium silicate membranes of this invention are useful forgas separations as described above, the use of titanium silicatemembranes in liquid separations is also part of this invention.Non-limiting examples of liquid-liquid separations, which can beachieved by the titanium silicate membranes of this invention includethe removal of organic, metallic and inorganic salt compounds fromaqueous solution. For example, the porous, crystalline titanium silicatemembranes can be used to separate halogenated solvents such as chloro-,fluro-, chlorofluro- or other halogenated hydrocarbons such as may beused in aqueous inks, paints, dry cleaning solvents, and general purposesolvents, etc., so as to recover the valuable solvent components and/orremoval of contaminant halogenated organics from aqueous streams.Likewise, non-polar organic components such as aliphatic and aromatichydrocarbons, such as gasoline components, e.g. C₅₊ aliphatics andbenzene can be removed from aqueous solution or from each otherdepending upon the size of the pores of the titanium silicate used. Theseparation of polar organic molecules, such as methanol, ethanol fromwater has important commercial applications. A particularly usefulseparation involving titanium silicate membranes of the presentinvention is the removal of methyltertiarybutylether (MTBE) fromcontaminated ground water such as resulting from the leakage of gasolinestorage tanks. FIG. 3 illustrates the separation of methanol (4.1 Å),ethanol (4.7 Å) or MTBE (6.6 Å) from water (2.7 Å) using an ETS-4 or CTSmembrane. The removal of anionic metallocompounds, such as complexes ofchromium, selenium and arsenic can be achieved and result in the removalof contaminant metals from ground water and as well provide separationof such metallocompounds from industrial aqueous streams to provideclean aqueous effluents as well as provide recovery of precious metalconstituents. Similarly, the titanium silicate membranes can be used toremove inorganic salts such as sodium chloride, metal sulfates, etc.from ground water and manufacturing streams such as from paper orpaperboard making.

A particularly useful titanium silicate would be ETS-4 which has anaverage pore diameter of about 4 microns which will allow water havingthe size of 2.7 Å to pass through while retaining larger molecules.Importantly, ETS-4 can be converted to CTS-1 so as to control the poresize from a maximum of 4 Å to about 3 Å, small enough to retaincontaminant or recoverable species and allow such species to beseparated from aqueous solution. Importantly, the titanium silicatemembranes of this invention readily allow the passage of the polar waterspecies therethrough, unlike the conventional aluminosilicate molecularsieves which ionically attract and hold polar species such as water.

EXAMPLE 1

A titanium silicate membrane formed of ETS-4 was formed in the followingmanner. 0.29 grams of TiO₂ was pressed into a disk with a diameter of 13mm and a thickness of 1 mm under 6 tons of pressure. The TiO₂ is P25(Degussa) and contains 76 wt. % anatase and 24 wt. % rutile.

A synthesis liquid was made from:

3.83 grams N-brand sodium silicate

0.60 grams 50% NaOH solution

3.04 grams deionized water

The solution of synthesis liquid was mixed well and placed in a Teflonautoclave liner (capacity about 15 ml.). The titanium dioxide disk wassubmerged in the solution and the autoclave sealed. The titanium dioxidedisk was submerged in the solution for sixteen hours under autogenouspressure at an oven temperature of 225° C. After removal of the diskfrom the solution, the disk was washed with hot water and dried at 100°C. XRD of the disk showed that it was formed of ETS-4.

EXAMPLE 2

An ETS-4 membrane similar to the one formed in Example 1 is used toseparate an ethanol/water mixture. Permeation measurements are carriedout at room temperature using the membrane submerged within an enclosedpermeation cell. Recirculating flow is provided to ensure good mixingnear the membrane surface. For permeation measurements at a given feedcomposition, three fractions of permeate are collected for at least sixhours and up to twelve hours. The feed and permeate compositions areanalyzed by gas chromatography. In the ethanol/water system, variouscompositions in the range of 10 to 90% water content are investigated.Selectivities for water in ethanol (α_(w/e)) are calculated using thefollowing equation (1). All selectivities α are between 200 to 400showing selective passage of water through the membrane. Increasing thetemperature increases water flux through the membrane without a decreasein selectivity. $\begin{matrix}{\alpha_{w/e} = \frac{( {{wt}\quad \% \quad {{water}/{wt}}\quad \% \quad {ethanol}} )_{permeate}}{( {{wt}\quad \% \quad {{water}/{wt}}\quad \% \quad {ethanol}} )_{feed}}} & (1)\end{matrix}$

Once given the above disclosure, many other features, modifications, andimprovements will become apparent to the skilled artisan. Such otherfeatures, modifications, and improvements are, therefore, considered tobe a part of this invention, the scope of which is to be determined bythe following claims.

We claim:
 1. A method for removing organic and/or inorganic contaminantsfrom an aqueous feed stream comprising contacting said aqueous feedstream with a membrane formed from a porous, crystalline titaniumsilicate, recovering a permeate rich in water and forming a retentatehaving a contaminant concentration greater than said aqueous feed streamsaid titanium silicate composed of six oxygen-coordinates octahedralwhich are connected three dimensionally by tetrahedral silicone oxideunits or bridging titanium silicate units.
 2. The method of claim 1,wherein said porous crystalline titanium silicate is selected from thegroup consisting of ETS-4, ETS-6, ETS-10, ETAS-10 and CTS-1.
 3. Themethod of claim 2, wherein said porous crystalline titanium silicate isETS-4.
 4. The method of claim 3, wherein said ETS-4 is ion-exchangedwith barium, calcium, strontium or mixtures thereof.
 5. The method ofclaim 2, wherein said porous crystalline titanium silicate is CTS-1. 6.The method of claim 1, wherein said porous crystalline titanium silicatehas an average pore size of up to 4 Å.
 7. The method of claim 1, whereinsaid contaminant is an alcohol.
 8. The method of claim 7, wherein saidalcohol is ethanol.
 9. The method of claim 1, wherein said contaminantis a halogenated solvent.
 10. The method of claim 1, wherein saidcontaminant is an aliphatic hydrocarbon having five or more carbonatoms.
 11. The method of claim 1, wherein said contaminant comprises anaromatic compound.
 12. The method of claim 1, wherein said contaminantis MTBE.
 13. The method of claim 1, wherein said contaminant comprisesmetallic compounds.
 14. The method of claim 13, wherein said metalliccompounds include arsenic.
 15. The method of claim 13, wherein saidmetallic compound comprises chromium.
 16. The method of claim 1, whereinsaid contaminant includes inorganic salts.
 17. The method of claim 16,wherein said inorganic salts include chlorides, sulfates or sulfites.18. The method of claim 1, wherein said aqueous feed stream comprisesground water.
 19. The method of claim 1, wherein said aqueous feedstream comprises an industrial process stream.
 20. The method of claim1, further comprising recovering said contaminant.